Plant Communities of the World 279 



separated, a subalpine Pinetum aristatae from 3600 to 3400 

 meters altitude and the Piceetum engelmanni from 3400 to 

 2700 meters. In the same belt in California, dominants are Pinus 

 jeffreyi and Abies magnifica from 2500 to 2200 meters altitude. 

 To the lower part of that belt probably belongs also the Wash- 

 ington forest, between 1400 and 600 meters altitude, of Pinus 

 monticola and various species of Abies. In the more arid area 

 Pinus ponderosa forms forests around 1000 meters altitude in 

 Washington and from 1900 to 2700 meters in Colorado. Moister 

 areas of the same belt are governed in California from 1500 to 

 2200 meters altitude by Abies concolor (in these forests are the 

 "Big Tree" groves). This forest prevails in Arizona around 2400 

 meters altitude. In the same belt live enormous Pseudotsuga 

 forests, but the Douglas fir spreads so widely by reason of biotic 

 factors that its forests derive from diverse causes. The lowest 

 parts of oceanic Washington have been mentioned in the dis- 

 cussion on Laurisilvae. 



A remarkable phenomenon occurs at the tree limit in the 

 Cascades. While the ascending crests are clad with trees, per- 

 petual snow lies on open flat places. In these mountains the 

 climatic snow limit lies at 2000 meters altitude, the tree limit at 

 2300 meters. This brings about the very interesting situation 

 that these important lines not only are near each other, but 

 actually transgrade. In the Swiss Alps the forest limit remains 

 at 700 to 1000 meters below the perpetual snow limit; but in the 

 very oceanic climate of Washington State, forests can thrive 

 above the cUmatic snow-line, and its alpine vegetation, which 

 only begins above the forest limit, thrives even higher. Phanero- 

 gams are known there up to 3200 meters altitude, that is, 1200 

 meters above the snow limit. A part of the subalpine belt and 

 the whole of the alpine belt fall here into the niveal belt. We 



